JP3894685B2 - Method for producing oxide garnet single crystal film - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for manufacturing an oxide garnet single crystal film used in magneto-optical elements such as optical isolators and optical switches.
[0002]
[Prior art]
Conventionally, when a magnetic garnet film with a film thickness of 30 μm or more is grown on a non-magnetic garnet single crystal substrate by liquid phase epitaxy, distortion occurs due to the difference in lattice constant between the single crystal substrate and the garnet epitaxial film. During the growth of the film or during the cutting process after the growth, there was a case where a crack was generated and the film was damaged. This strain is known to cause birefringence in the magnetic garnet film.
Various proposals have been made to avoid the occurrence of such distortion. For example, Japanese Patent Laid-Open No. 57-160105 proposes that the lattice constant difference Δa between the single crystal substrate and the magnetic garnet film be 0.001 mm or less. is doing.
[0003]
JP-A-62-143893 discloses that when a garnet film is grown on a non-magnetic garnet substrate by a liquid phase epitaxial method, a composition change of the melt occurs, and accordingly, the relationship between the growth temperature and the lattice constant of the epitaxial crystal. Therefore, it has been proposed to change the growth temperature so that the lattice constant of the epitaxial crystal becomes equal to the lattice constant of the substrate.
However, when such a garnet film is grown by the liquid phase epitaxial method, there is a problem that the substrate is often broken during the growth.
[0004]
In order to solve these problems, Japanese Patent Publication No. 7-69522 discloses that a garnet layer is formed of at least a first layer and a second layer, and the first layer has a lattice constant of this layer at a first temperature of the substrate. Having a first composition selected to be approximately equal to the lattice constant and greater than the lattice constant of the substrate at a second temperature higher than the first temperature, and the second layer has a lattice constant of the layer A magneto-optic isolator device having a second composition selected to be smaller than the lattice constant of the substrate at the first temperature and smaller than the lattice constant of the first layer at the second temperature is proposed.
However, in this proposal, it is difficult to grow the substrate without breaking during the growth of the garnet magnetic thick film having a thickness of 200 μm or more on the large-diameter substrate having a diameter of 2 inches or more. Further, when a plurality of first and second garnet layers are provided, birefringence occurs in the garnet magnetic film, and there is a problem that the extinction ratio decreases when this garnet film is used as a magneto-optical element.
[0005]
On the other hand, in Japanese Patent Laid-Open No. 4-139093, in the growth of a bismuth (Bi) -substituted rare earth iron garnet single crystal film by a liquid phase epitaxial method, the fluctuation of the lattice constant in the growth direction of the single crystal film is indicated as Disclosed is a method for producing a bismuth-substituted rare earth iron garnet single crystal film that grows the film while lowering the melt temperature during the growth of the single crystal film so that the constant is controlled to ± 0.003 mm or less. ing.
[0006]
Further, in JP-A-5-330993, paying attention to the value of the difference Δa between the lattice constant of a garnet single crystal film having a thickness of 50 μm or more grown by a liquid phase epitaxial method and the lattice constant of a single crystal substrate, A garnet single crystal film in which −0.004 Å <Δa <−0.001 Å is disclosed at all points inside and outside the garnet single crystal film.
Further, Japanese Patent Laid-Open No. 6-92796 discloses a single crystal film obtained by increasing the lattice constant deviation from the substrate side toward the growth direction in a single crystal film obtained by epitaxial growth.
[0007]
However, in these methods, since the thermal expansion coefficients of the single crystal substrate and the garnet magnetic film are different, the film and the substrate are distorted during the growth of the garnet epitaxial film. When epitaxial films are formed on both sides of the substrate, stress remains in the epitaxial film without releasing strain during the cooling process after the epitaxial film is formed, causing birefringence or cracking in the epitaxial film. The distortion is released.
[0008]
Further, in order to cut out the epitaxial film on both sides of the single crystal substrate and then cut it out as a magneto-optical element, it is necessary to cut the single crystal substrate to be sliced from the lateral direction (hereinafter referred to as slice cutting) and to divide it. Normally, an inner peripheral cutting machine is used for this slice cutting, but if the stress remains in the epitaxial film without releasing the strain in the cooling process after the epitaxial film is formed, the internal cutting is performed as the cutting proceeds. The cut object is vibrated slightly by the peripheral blade, and the stress gradually increases, and a crack is generated in the garnet film formed on both sides of the substrate.
[0009]
[Problems to be solved by the invention]
An object of the present invention is to grow an oxide garnet single crystal film having a thickness of 200 μm or more on both sides of a large-diameter single crystal substrate having a diameter of 2 inches or more without breaking the substrate, and an epitaxial film is formed on both sides. Another object of the present invention is to provide a method of manufacturing an oxide garnet single crystal film that can be easily separated and divided without slicing and cutting the single crystal substrate and contributes to an improvement in the yield of magneto-optic chip.
[0010]
[Means for Solving the Problems]
As a result of intensive investigations in view of the above problems, the present inventors have found that when cooled from the growth temperature, near the room temperature, due to the difference in lattice constant Δa between the single crystal substrate and the garnet single crystal film, Stress is accumulated in the garnet single crystal film. Since the accumulated stress is distributed to the target toward the garnet single crystal film grown on both sides of the substrate with the single crystal substrate as a boundary, by appropriately changing the Δa value with the growth of the garnet single crystal film, The substrate was grown without breaking, and the conditions under which shear stress was applied to tear the substrate in the lateral direction were found, and the present invention was completed.
[0011]
That is, the method for producing an oxide garnet single crystal film according to the present invention is a liquid phase epitaxial method in which an oxide garnet single crystal film having a thickness of 200 μm or more is grown on both surfaces of a single crystal substrate. The lattice constant of the film is a _f1 , the lattice constant of the single crystal film gradually increases as it grows, the maximum value of the lattice constant of the single crystal film at the initial stage of growth is a _f2 , and the lattice constant of the single crystal film at the end of growth is If a _f3 and the lattice constant of the single crystal substrate is a _S ,
−0.017Å ≦ a _f1 −a _S <−0.006Å,
_{-0.014 Å <a f2 -a S ≦} -0.003 Å,
-0.014 Å ≦ a _f3 −a _S < -0.003 Å
It is characterized by controlling the liquid phase epitaxial conditions. Further, it is preferable that a _f2 and a _f3 are in a range satisfying −0.001Å <a _f2 −a _f3 <0.002Å.
[0012]
In growing the oxide garnet epitaxial film, the epitaxial conditions such as the melt temperature and the temperature lowering rate may be appropriately controlled so that a _f1 , a _f2 , and a _f3 of the epitaxial film are in the above ranges.
Differences in lattice constants a _f1 −a _S , a _f2 −a _S , and a _f3 −a _S between the epitaxial film and the substrate are respectively expressed as _{Δa 1} , Δa ₂ , and Δa ₃ in this order, and a _f2 − a _f3 is denoted as Δa ₂₃ . These lattice constants are measured values at room temperature.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
This will be described in further detail below. The oxide garnet single crystal film obtained by the production method of the present invention mainly has a first layer having a large lattice constant difference Δa ₁ between the epitaxial film and the substrate at the start of growth of -0.017 to -0.006 Lattice constant difference Δa ₂ between the epitaxial film and the substrate at the initial growth stage , Difference in lattice constant between the epitaxial film and the substrate to foster early-growing ends △ a _2, △ a _3, respectively, △ a ₂ But greater than -0.014 Å -0.003 Å or less, △ a ₃ There has a two-layer structure of the second layer of ~ -0.003 Å smaller ranges -0.014 Å or more. The thermal expansion coefficient of the epitaxial film is larger than the thermal expansion coefficient of the substrate at the growth temperature. However, since Δa ₁ of the first layer is large, the tensile stress applied to the substrate is reduced during the first layer growth, and the substrate is broken. Never do. Further, since the second layer also has large Δa ₂ and Δa ₃ , the substrate does not break even during the growth of the second layer having a film thickness exceeding 200 μm. On the other hand, when Δa ₂ and Δa _{3 of} the second layer are smaller than the above values, the substrate does not break at a film thickness of 200 μm or less, but when a thick film exceeding 200 μm is grown, the substrate is grown during the growth. Breaks.
[0014]
Furthermore, when the epitaxial film grown to a thickness of 200 μm or more is cooled from the growth temperature, the first layer has a much larger | Δa | than the second layer. The first layer receives a strong tensile stress, the second layer receives a weak compressive stress, and a shear stress acts so as to tear the substrate in the lateral direction, and the substrate is divided into two epitaxial films each having a substrate. At this time, when the crack extends from the nail holding the substrate on the side surface toward the inside of the substrate, the crack propagates in a lateral direction from the side surface of the substrate, and the substrate is laterally cracked as if it was peeled off. As a result, the shear stress is released. Since the laterally cracked epitaxial film with a substrate has a large | Δa |, since the stress is relieved by deformation to a slightly concave shape, the epitaxial film itself is less likely to crack.
Further, since the substrate is naturally peeled in the lateral direction due to the shear stress acting on the substrate in this way, it is not necessary to slice and cut the substrate on which the epitaxial film is formed from this side surface.
[0015]
The value of the difference △ a ₁ in the lattice constant, in order to generate transverse cracks in the substrate, it is necessary to lower than -0.006A, also when smaller than -0.017 Å, interface between the substrate and the epitaxial film Therefore, it is necessary that the thickness be −0.017 mm or more because a fine crack nest is formed and this fine crack propagates into the epitaxial film during processing and the epitaxial film is decomposed. The values of Δa ₂ and Δa ₃ are combined with the initial epitaxial film having a film thickness of about 40 μm to cause lateral cracks in the substrate, and so that no cracks are generated in the substrate during epitaxial film growth. Therefore, if it is smaller than -0.014 mm, on the other hand, the substrate is divided into fine regions during cooling and lateral cracking occurs, and the yield as an epitaxial chip is lowered. -0.014 Å or more is necessary. In particular, if -0.001 Å <△ a ₂₃ <0.002 Å, the number of chips that can be used as a magneto-optical element without cracking the epitaxial film during processing is extremely small when the substrate is laterally cracked. Is preferable because it greatly increases.
[0016]
During cooling, the single crystal substrate receives the shear stress generated at the epitaxial film interface from both sides of the substrate, and the substrate is laterally cracked so as to tear from the side. At this time, the stress caused by the difference between the lattice constants of the substrate and the epitaxial film generated during the growth is released, and the distortion of the epitaxial film layer disappears.
As a result, in order to cut out the oxide garnet single crystal film grown on both surfaces of the substrate, the step of slicing the substrate in the lateral direction is not necessary.
[0017]
The oxide garnet substrate used as a starting material may be a conventionally known one, such as gadolinium gallium garnet (GGG), neodymium gallium garnet (NGG), samarium gallium garnet (SGG), NOG (made by Shin-Etsu Chemical Co., Ltd., trade name) in which a part of the ion is replaced with Ca, Mg, Zr, and the like.
The oxide garnet single crystal film grown on these oxide garnet substrates by liquid phase epitaxy may be any industrially useful one, including (BiEuTb) ₃ (FeGa) ₅ O ₁₂ , (BiY) ₃ Fe ₅ O _{12 and the} like are exemplified, but the thickness of these epitaxial films is preferably 200 μm or more, which can provide a sufficient Faraday rotation angle with near-infrared wavelength light.
[0018]
As for the depth position of the substrate in the melt, the temperature of the melt changes abruptly near the melt surface, so that a temperature difference occurs between the upper surface and the lower surface of the substrate, and the lattice differs between the upper surface and the lower surface. A constant epitaxial film grows, which is not preferable. For this reason, it is better to form an epitaxial film at a depth of at least 10 mm away from the melt surface, thereby growing an epitaxial film having the same lattice constant and the same film thickness on the upper and lower surfaces of the single crystal substrate. Can do.
[0019]
【Example】
Next, examples of the present invention and comparative examples will be given. The lattice constant is measured by a precision lattice constant measuring apparatus (bond method).
[Example 1]
As a single crystal substrate, 1.2 mm polished both sides of single crystal NOG (previously trade name) in which a portion of the cation of a GGG single crystal with a 2 inch diameter lattice constant of 12.496 mm is replaced with Ca, Mg, Zr A thick wafer was prepared.
As the metal oxide to form an oxide garnet epitaxial film, and _{Bi 2 O 3 977.38g, Eu 2} O 3 31.489 g, Tb 4 O 7 13.929g, Fe 2 O 3 122.86 g, Ga 2 O 3 35.272g, flux Weigh 936.36 g of PbO and 341.72 g of B ₂ O ₃ as ingredients, heat and melt them in a platinum crucible, set the previously prepared single crystal substrate in a platinum holder, and match the substrate lattice constant at room temperature. At a temperature 15 ° C higher than the temperature at which the epitaxial film grows (approx. 750 ° C), the substrate is immersed in a position 20 mm below the melt surface, and growth of the garnet single crystal thin film is started while rotating the substrate. After the temperature was lowered by about 7 ° C., an epitaxial film was grown for 60 hours at a temperature lowering rate of 1 ° C. over about 7 hours.
When this was taken out of the epitaxial furnace and cooled, the single crystal substrate was peeled laterally by the accumulated stress, and an epitaxial film having a thickness of 600 μm was formed on each substrate.
[0020]
Next, this single crystal substrate was washed in a 10% nitric acid solution to wash off the flux components in the epitaxial film, and the epitaxial film was examined. As a result, the oxidation was represented by (BiEuTb) ₃ (FeGa) ₅ O ₁₂ It was a garnet. When the lattice constant of the epitaxial film was examined in detail along the growth direction of the film and the difference Δa value (mismatch) with the lattice constant of the substrate was examined, both surfaces of the epitaxial film showed the values shown in FIG.
FIG. 1 is a graph showing a change in mismatch amount Δa in the film thickness direction. As shown in the figure, Δa ₁ at the start of growth is large at −0.009 mm, and the mismatch gradually decreases with growth, and Δa ₂ is −0.004 mm at the initial stage of growth with a film thickness of about 40 μm. As the film thickness approaches the growth end film thickness, the mismatch increased again, and Δa ₃ (mismatch) at the end of the growth period was -0.0045 mm. At this time, Δa ₂₃ was 0.0005%.
[0021]
Next, the oxide obtained by lapping the single crystal substrate on which this epitaxial film was grown with GC # 600 green carbon to eliminate irregularities, and mirror polishing to a thickness of 425 μm with a polishing machine using colloidal silica and cloth When the garnet epitaxial film was cut into 2 mm squares with a dicing saw, 530 Faraday rotators as magneto-optical elements useful for optical isolators were obtained.
[0022]
(Example 2, Comparative Examples 1-3)
The metal oxide composition of the single crystal substrate and the epitaxial film grown on this surface was the same as in Example 1.
In growing the epitaxial film, epitaxial conditions such as the melt temperature and the cooling rate were appropriately controlled to obtain epitaxial films having different lattice constant differences Δa between the epitaxial film and the substrate. About each of these epitaxial films, it shows below as Example 2 and Comparative Examples 1-3.
[0023]
[Example 2]
This epitaxial film is an oxide garnet represented by (BiEuTb) ₃ (FeGa) ₅ O ₁₂ and has a film thickness of 620 μm. Δa shows the same tendency as in FIG. 1, and the mismatch at the start of growth is Δa ₁ = large as -0.007A, gradually decreases as development, mismatches in the early stages of growth film thickness of about 40μm minimal △ a ₂ = -0.003Å next, as close to the growing ends thickness mismatches increases again, △ a ₃ = -0.0053cm. At this time, Δa ₂₃ = 0.0023cm.
Although the single crystal substrate peeled off in the lateral direction, some cracks were also formed in the epitaxial film, and the epitaxial film was divided into a plurality of pieces with this crack as a boundary. When Faraday rotators were produced from this oxide garnet epitaxial film in the same manner as in Example 1, 485 pieces were obtained.
[0024]
[Comparative Example 1]
This epitaxial film has a thickness of 640 μm and is an oxide garnet represented by (BiEuTb) ₃ (FeGa) ₅ O ₁₂ , and the Δa value shows the same tendency as in FIG. 1, but the mismatch at the start of growth is maximum. △ a ₁ = -0.005 mm, gradually decreases with growth, and the mismatch at the initial stage of growth with a film thickness of about 40 μm is the minimum Δa ₂ = -0.001 mm, and the mismatch increases again as the film thickness approaches the end of growth. Δa ₃ = −0.0025 Å. Δa ₂₃ at this time was 0.0015 mm.
No peeling in the lateral direction was observed on this single crystal substrate, and this oxide garnet epitaxial film was broken into pieces during processing, and the Faraday rotator as in Example 1 was not obtained.
[0025]
[Comparative Example 2]
This epitaxial film has a thickness of 630 μm and is an oxide garnet represented by (BiEuTb) ₃ (FeGa) ₅ O ₁₂ , and the Δa value shows the same tendency as in FIG. 1, but the mismatch at the start of growth is maximum. △ a ₁ = -0.0055 mm, it gradually decreases as it grows, and the mismatch at the initial stage of film growth of about 40 μm becomes the minimum Δa ₂ = -0.002 mm, and the mismatch increases again as it approaches the film thickness at which growth ends. Δa ₃ = −0.005cm. At this time, Δa ₂₃ = 0.003 Å.
No lateral peeling was observed on this single crystal substrate. When the single crystal substrate was sliced and divided from the lateral direction using an inner peripheral cutting machine, a crack was generated in the epitaxial film, and when a Faraday rotator was produced in the same manner as in Example 1, only 250 pieces were obtained. There wasn't.
[0026]
[Comparative Example 3]
This epitaxial film is an oxide garnet represented by (BiEuTb) ₃ (FeGa) ₅ O _{12. The} film thickness is 600 μm, and the Δa value shows the same tendency as in FIG. 1, but the mismatch at the start of growth is the largest. △ a ₁ = -0.019 Å, it gradually decreases with growth, and the mismatch at the initial stage of growth of about 40 μm is the smallest △ a ₂ = -0.014 、, and the mismatch becomes larger again as the growth finishes near the film thickness. Δa ₃ = −0.0145 Å. At this time, Δa ₂₃ = 0.0005cm.
Although this single crystal substrate was peeled off in the lateral direction, the epitaxial film was also cracked, and the epitaxial film was finely divided with this crack as a boundary. When Faraday rotators were produced from this oxide garnet epitaxial film in the same manner as in Example 1, only 50 pieces were obtained.
[0027]
【The invention's effect】
The method for producing an oxide garnet single crystal film according to the present invention comprises forming an oxide garnet single crystal film having a thickness of 200 μm or more on both sides of a large-diameter single crystal substrate having a diameter of 2 inches or more without breaking the substrate during growth. When the substrate is cooled from the growth temperature to room temperature, the substrate is torn by the shear stress from the lateral direction, and mechanical slice cutting at the substrate portion is unnecessary, and the yield of the chip for the magneto-optic element is drastically increased. Improved.
[Brief description of the drawings]
FIG. 1 is a graph showing changes in lattice constant difference (Δa) with respect to the film thickness direction of an epitaxial film.

Claims (2)

When growing an oxide garnet single crystal film having a film thickness of 200 μm or more on both sides of a single crystal substrate by liquid phase epitaxy, the lattice constant of the garnet single crystal film at the start of the growth is a _f1 , and the lattice of the single crystal film as it grows The maximum value of the lattice constant of the single crystal film at the initial stage of growth when the constant gradually increases is a _f2 , the lattice constant of the single crystal film at the end of the growth is a _f3, and the lattice constant of the single crystal substrate is a _S Then
−0.017Å ≦ a _f1 −a _S <−0.006Å,
_{-0.014 Å <a f2 -a S ≦} -0.003 Å,
-0.014 Å ≦ a _f3 −a _S < -0.003 Å
The manufacturing method of the oxide garnet single crystal film characterized by controlling liquid phase epitaxial conditions so that it may become. 2. The method for producing an oxide garnet single crystal film according to claim 1, wherein the lattice constant a _f2 and the lattice constant a _f3 have a relationship of −0.001Å <a _f2 −a _f3 <0.002Å.

Images